This program project in cellular developmental neurobiology is concerned with determining genetic and environmental factors which influence the form and function of neurons. In each of several projects there is consideration of the hypothesis that during development there is a continuous interaction between the determinants of form and function, and that these interactions are essential for the realization of the properties of the final differentiated neuron. Some of the research deals with the developing nervous system in frog tadpoles (the lateral motor column of the spinal cord; the influence of limb regeneration, of thyroid hormones, and specific nerve cell-muscle group interactions) and Drosophila (electrical properties and morphology of identified neurons in the ventral ganglion, with use of temperature-sensitive mutants that produce a block of axonal conduction). Other systems involve development of axonal and dendritic branching patterns of regenerating neurons in the snail, and the redevelopment of specific nerve-muscle connections in the cockroach leg following axonal section. Determination and development of synaptic properties in the giant squid synapse and the use of chemotactic behavior in paramecium as a model system for describing sensory-transduction pathways are the remaining projects.